Molecular drag pumping stages produce pumping action by momentum transfer from a fast-moving surface (moving at speed comparable to thermal speed of the molecules) directly to gas molecules. Generally, said pumping stages comprise a rotor and a stator cooperating with each other and defining a pumping channel therebetween: collisions of gas molecules in the pumping channel with the rotor rotating at a very high speed cause gas in the channel to be pumped from the inlet to the outlet of the channel itself.
With reference to FIG. 1, between 1920-1930 Karl Manne Georg Siegbahn developed a molecular pumping device 10, wherein the pumping action is obtained through the cooperation of a rotor disk 20 having smooth surfaces integral with a rotating shaft 30 with a pair of stator bodies 40, 50, each facing a rotor disk surface and provided with a corresponding spiral-shaped groove 60 open towards the respective surface of the rotor disk and defining therewith a corresponding pumping channel.
The Siegbahn patent GB 332,879 discloses an arrangement of the above-mentioned kind. The gas to be pumped, entering through an inlet 70 at the outer periphery of each pumping groove flows in both spiral channels in centripetal direction, i.e. from the outer periphery towards the pumping grooves, as indicated by arrows CP. In this case, two spiral pumping channels in parallel are to be considered, the gas flows in both channels in centripetal direction.
According to Siegbahn, in order to control the resistance of the gas pumped through the spiral channels 60, the cross-section area of these channels is reduced from the outer periphery of the stator bodies towards their center, in accordance with the reduction of the tangential speed of the disk, in the direction of the gas flow.
U.S. Pat. No. 6,394,747 (M. Hablanian) discloses a vacuum pump having reduced overall size and weight utilizing for these purposes a pair of Siegbahn-type pumping stages connected in series rather than in parallel.
According to U.S. Pat. No. 6,394,747 disclosure, a rotor disk having smooth surfaces is placed between a first stator body and a second stator body, each of these stator bodies are provided with a spiral groove open towards the respective surface of the rotor disk and defining therewith a corresponding pumping channel. At the beginning, the gas to be pumped flows between the first stator body and the rotor disk in centrifugal direction, from the center to the outer periphery of the rotor disk, and then between the second stator body and the rotor disk in centripetal direction, i.e. from the outer periphery to the center of the rotor disk.
The cross-section area of the groove defining the pumping channel in the first stator disk—wherein the gas flows in centrifugal direction—is reduced from the centre to the outer periphery, while the cross-section area of the groove defining the channel in the second stator disk—wherein the gas flows in centripetal direction—is reduced from the outer periphery to the centre.
In both channels, the cross-section area of the groove defining the pumping channel is reduced concordantly with the advancing direction of the flow of the gas that is pumped through the channel itself. In this way, U.S. Pat. No. 6,394,747 aims at optimizing the pumping speed and the compression ratio.
In known Siegbahn-type pumping stage, having the above-mentioned geometric configuration, the volumetric internal channel speed (L/s), given by the product of the channel cross-section area and half the rotor velocity normal to the aforesaid area, is reduced concordantly with the gas flow direction. This may constitute a drawback in applications with high gas flow rates, since it generates the risk of internal compressions and successive re-expansions and corresponding power losses.
The main object of the present invention is to provide a centripetal pumping stage for vacuum pump, which allows to overcome the above-mentioned drawback and to reduce power losses, when several stages are connected in series. This and other objects are achieved by centripetal and centrifugal pumping stages of the present invention.